1. Field of the Invention
The present invention relates to an optical cable in which a plurality of optical fibers are received, and a desired one of the optical fibers in easy to take out and branch at an intermediate portion of the cable, and a manufacturing method thereof.
2. Description of the Related Art
Recently, it has come to be necessary to lay optical fibers to users as in seen in general residential areas. In laying optical fibers, it is required to reduce the number of connection points of the optical fibers to the minimum in order to maintain the transmission quality good, and it is also required to reduce the construction cost of an optical communication network. In order to satisfy those requirements, there is a method wherein an optical cable in which excessive optical fibers are received and the optical fibers can be taken out at the intermediate portion of the cable easily is used, so that in an existing optical cable which is in use, a required number of ones of excessive optical fibers received therein can be branched and taken out at desired portions of the optical cable other than its terminals in order to cope with newly produced necessity.
Conventionally, there has been known a so-called SZ slotted optical cable, that is, an optical cable having a reverse-lay unit core, as an optical cable in which it is easy to take out optical fibers at an intermediate portion, as disclosed, for example, in Unexamined Japanese Patent Publication No. Sho-63-301911, and the like. This optical cable has a structure in which the rotating direction of a plurality of spiral grooves cut in a grooved spacer is inverted at predetermined periods, and optical fibers are received in these spiral grooves respectively. A costing layer formed by an extrusion coating of synthetic resin may be provided on the outside of the grooved spacer, and a pressing winding of, for example, non-woven fabric or the like, a pressing winding of a filament-like body such as a nylon yarn spirally wound at intervals, or an aluminum layer may be provided under the coating layer. Herein, those layers are called a sheath collectively. That is, it can be said that the structure in which optical fibers are received in SZ slots (grooves) and a sheath is given thereon has been known.
In such a branching working, some of the optical fibers in the optical cable are taken out while some of the optical fibers are in use for transmission. In a conventional optical cable, there has been a problem that not only specific optical fibers which are required to be taken out but also other optical fibers which are in use may pop out in the inverted portion of the grooves when the sheath is removed in the branching working. If the popped-out optical fibers which are in use touch a neighboring object, a change of loss is produced in optical signals which are being transmitted. Consequently, there has been a problem that the transmission quality of the optical fibers which are in use deteriorates.